Pseudo-hexagonal packing

These phases very often share, aside from a relatively high disorder, another important feature with the bundle-like pre-crystalline entities that we discussed up to this point. This feature is hexagonal or pseudo-hexagonal packing of the polymer chains [61]. In this respect molecular order at a local level is similar, although in the thermotropic mesophases we are now discussing disorder is even more dynamic than in previous cases. Indeed they are essentially characterized by a high entropy. 

Figure 1.6. Schematic diagram of four unit cells of /rons-polyacetylene showing pseudo-hexagonal packing and in phase and 180° out of phase relationships of the double and single bonds between nearest neighbours. (Reproduced from ref. 59 with kind permission. Copyright (1987) Elsevier Science Ltd, Kidlington 0X5 IGB, UK.)...

Main-chain LCPs that exhibit nematic order tend towards pseudo-hexagonal packing in the liquid-crystalline state. Wide-angle X-ray-diffraction... 

The X-ray fiber diffraction patterns of form II show two equatorial reflections and a diffuse meridional 002 reflection [66,165]. The equatorial reflections indicate a pseudo-hexagonal packing of chain axes with a = 4.88 A and c = 4.65 A [66]. During the transition from form I into form II, the c axis shrinks by 4% while the distance between the chain axes increases by 7.5% and the volume per chain in the crystalline state increases by 9% [8]. 

The structure of rutile, Ti02-x, is tetragonal, with a pseudo-hexagonal packing of oxygen ions in the (001) plane (space group P42/mnm). Rutile is commonly oxygen deficient. It is an ore for titanium and occurs in quartz veins in metamorphic rocks and as placer deposits. 

Fig. 9 Two arrangements of helical chains, represented as continuous screws, packing in a hexagonal or a pseudo-hexagonal mode. In a the three screws are isomorphous in b the screw B is enantiomorphous with respect to both A and C and the overall packing is less satisfactory. Reproduced with permission from [13]. Copyright 1998 Am Chem Soc...

Up to 2% authigenic kaolinite and dickite occur in host rocks as loosely packed booklets of pseudo-hexagonal crystals (5-15 pm). They occur on authigenic quartz overgrowths and on partly dissolved feldspar grains. 

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